Less than 10 hours later, a 4.5-magnitude quake struck two miles from the same location.

“In the case of the first event (Tuesday) night, here in Pasadena, we got about nine seconds warning before the strongest shaking was felt here,” said Douglas Given from the U.S. Geological Survey. “In the case of the second quake, it was a little bit less…about four seconds warning.”

Experts said this prototype system is the first to ever pick up quakes before the ground started shaking.